Plants are colored according to the pigmentation given to them in their family of flower (genome), called flavanoids.
Here are a few examples:
-Flowers, usually, Red, Pink, Blue and Purple have a pigment called Anthocyanins.
-Orange, Yellow and Red tints, like in carrots and tomatoes have pigments called carotenoids.
-All of the pigments of green you see in plants are called chlorophyll.
The pigments in the plants will actually reflect the colors from the UV wavelength. So chlorophyll a will reflect green and thus giving the plant it's green color.